There are two forms of tungsten carbide. Monotungsten carbide has the formula WC, and di-tungsten carbide has the formula W.sub.2 C. Of the two, WC has much greater commercial value and is used for the manufacture of various cemented carbide tools and structural components.
In the past, tungsten monocarbide powders have been produced mainly by two routes, the direct carburization process and the alumino thermit process (also known as the Macro Process).
In alumina thermit process, described in P. C. McKenna. U.S. Pat. No. 3,379,503 the mixture of tungsten ore concentrates and iron oxide are reduced by aluminum metal and simultaneously carburized by the use of carbon source such as calcium carbide and carbon. The reactants were added such that they can develop a self-sustaining exothermic reaction at a calculated temperature of about 2445.degree. C. In order to minimize the amount of undesirable carbide phases such as W.sub.2 C and the compounds of the type M.sub.3 W.sub.3 C, the process temperature must be kept in the range between 2482.degree. C. and 412.degree. C. To maintain the temperature within this range, it was found necessary to blend high grade tungsten ore concentrates with low grade tungsten ore concentrates which normally contain excessive amounts of Ti. Nb and Ta as impurities.
An improved version of the aluminothermit method of producing tungsten monocarbide powders is described in detail in U.S. Pat. No. 4.834.963. In this process, metallic iron is added to the reaction charge in quantities to control the calculated reaction temperature within the range of about 2411.degree. C. to about 2482.degree. C. Furthermore, it was found that this process can be controlled to produce macrocrystalline tungsten carbide powders with very low content of Ti, Ta and Nb and a narrow range of total carbon content.
The direct carburization process involves subjecting tungsten metal. tungstic acid, ammonium paratungstate or tungsten oxide powder to carburization with finally divided carbon at about 1400 to 1700 degrees centigrade Canadian patent No. 4,664,899 describes the production of WC from ammonium paratungstate (APT). The APT is first calcined at temperatures between 540.degree. and 620.degree. C. to produce blue tungsten oxide, which is then placed in molybdenum boats and reduced in dry hydrogen at 700.degree. to 900.degree. C. to form tungsten metal powder. The metal powder is then blended with carbon black and carburized at 1500.degree. C. in a graphite crucible. In another process described in the paper published by M. Miyake, A. Hara and T. Sho, "The Direct Production of WC from WO.sub.3 by using Two Rotary Carburizing Furnaces" in the Journal of Japan Society of Powder and Powder Metallurgy, Vol., 26. pp. 90, 1979, and carbon mix is pelletized and reacted in a series of two rotary furnaces The first furnace operates in nitrogen and allows the formation of a mixture W-W.sub.2 C-WC-C. The material is then fed directly into a second rotary furnace operating in H.sub.2 for final carburization at a higher temperature In addition to the problems associated with higher capital investments due to the use of two furnaces the process requires the strict control of the CO-CO.sub.2 partial pressure ratio.
A more recent method of producing WC from either WO.sub.3 or APT and carbon source is described in U.S. Pat. No. 4,664,899. The process consists of two steps. In the first step, WO.sub.3 (or APT) and a substoichiometric amount of carbon are mixed in either a mixer or a ball mill. The mixture is loaded in graphite boats and carburized in a nonreducing atmosphere. This results in a partially carburized mixture depleted of carbon near the powder bed surface (rich in W) and rich in WC and W.sub.2 C in the center of the bed. In the second step, the partially carburized mixture is removed from the furnace, sampled, and an appropriate amount of carbon powder is added. After blending, the mixture was carburized in a reducing atmosphere which is preferably hydrogen.
In the above cited processes involving the direct reduction of WO.sub.3 or APT to WC two major problems can be identified. First the tungsten carbide particle size is difficult to control due to water vapour deposition due to hydrogen which occurs in the carburizing powder bed. Also, to produce fine tungsten carbide powder, high hydrogen flow rates are required. Second, the complexity of the direct reduction and carburization reactions makes utilization of large boat loads very difficult. This is further aggravated by the low thermal conductivity of the powder bed in the furnace, which produces large thermal gradients between the center and the outer surfaces. This results in the center of the powder bed reacting at a later time. The CO.sub.2 gas liberated from the central portion of the bed must pass through the bed surface allowing the following reaction to occur. EQU CO.sub.2 +C.fwdarw.2CO
which leads to carbon depletion from the outer portions of the powder bed. This results in a center core in the powder bed containing high percentages of carbide (WC and W.sub.2 C), while the top surface of the bed contains primarily W and W.sub.2 C. Due to this the maximum output did not exceed 1.2-2.0 kg/h per furnace tube.
Evidently, the reduction of both WO.sub.3 and TiO.sub.2 to metals is controlled by the partial pressure of CO and CO.sub.2.